The present invention relates to electromagnetic filters, and more particularly to a microwave integated circuit filter.
At microwave frequencies, as at other frequencies, filters are used to select or reject bands of electromagnetic frequencies. For example, the information sought to be received is usually within a specified frequency range, and it is desirable to filter out extraneous frequencies which might otherwise appear as "noise" when the signals are decoded, or otherwise transformed. The effectiveness of a filter often depends on the effectiveness with which it rejects out-of-band frequencies.
One microstrip filter is disclosed in "Microstrip Bandpass Filter Using Degenerate Modes of a Microstrip Ring Resonator", by I. Wolff, Electronic Letter, June 15, 1972, Vol. 8, No. 12. Wolff indicates that two degenerate modes can be coupled if the symmetry of a resonator is disturbed. Wolff achieves asymmetry in two ways. In the first, one transmission line is directed obliquely to a second transmission line, both lines being coupled to the intermediate resonator element in a bandpass filter. In the second, a notch is cut into the resonator which differentially affects the two resonance modes. However, Wolff's arrangement does not provide clear practical advantages over available filters.